Ancient Volcanic Rocks on Vinalhaven Island
Vinalhaven Island is a large island in Penobscot
Bay, 1 hour and 15 minutes by ferry from Rockland. The central and
southern parts of Vinalhaven are underlain by a massive body of pink, medium-grained
to fine-grained granite. The Vinalhaven granite was quarried in the late
1800's and early 1900's as part of the famous Maine granite
industry that provided dimension stone and decorative stonework for
large buildings and bridges in Boston, New York, Chicago, and elsewhere.
The old quarries are inactive, and Vinalhaven today is a major lobster-fishing
town.
Figure 1 |
The geology of the northern part of Vinalhaven, from Middle Mountain
to Browns Head, is less famous than the granite but in some ways more interesting.
A variety of volcanic rocks give clues to an ancient past when this part
of the earth's crust was being formed. Collectively, these rocks have been
named the Vinalhaven Rhyolite, after the most common type of volcanic rock
there. Figure 1 shows a flow-banded rhyolite, a hard, black rock with thin
stripes. This photo was taken along North Haven Road north of Middle Mountain,
just before the radio tower. This kind of rock extends from the road southward
over Middle Mountain. Rhyolite is a volcanic rock that solidifies from
a very thick, paste-like lava. As it slowly moves up the volcanic vent,
the lava is stretched like taffy to produce the fine streaks and stripes. |
Figure 2 |
Figure 2 shows a volcanic breccia. Notice the angular rock fragments
that comprise this rock. Many of the fragments consist of flow-banded rhyolite
like the rock shown in Figure 1. Volcanic breccias are formed in explosive
volcanic eruptions during which lava and solid rocks of the volcanic vent
are blown apart into fragments of different sizes. Some of the rock fragments
in Figure 2 are several inches across and are too big too have been thrown
very far from the volcanic vent. |
Figure 3 |
Figure 3 shows a layered or stratified rock. As in Figure 2, the small
fragments consist mainly of rhyolite and other volcanic rock bits. But
an important difference is that the rock in Figure 3 has layers in which
the fragments have been sorted according to their size. Some layers consist
of pebble-sized fragments, while other layers have sand-sized fragments
(note lens cap for scale). This sorting indicates that the volcanic debris
has been washed down a slope by water and deposited in layers. This type
of rock is called a water-laid tuff. These deposits accumulate near active
volcanoes, but may have been carried some distance from the volcanic vent. |
Figure 4 |
The rock shown in Figure 4 also consists of layers of volcanic debris,
but one of these layers contains peculiar round balls called accretionary
lapilli. Close examination of the outcrop with a magnifying lens reveals
that some of these little mud balls are internally layered like an onion.
It is thought that accretionary lapilli form when an eruption spews a cloud
of very finely pulverized dust into the atmosphere. Then, if rain clouds
develop, the dust particles become stuck together and grow as they circulate
within the cloud, much like a hailstone grows. When the wet mud balls grow
big enough, they rain down to the ground, making a layer of mud balls like
the one seen in this photo. |
Figure 5 |
The schematic diagram in Figure 5 shows how the different rocks shown in
Figures 1-4 might have formed in a volcanic environment. Flow-banded rhyolite
solidifies directly from lava in the volcano. Explosive eruptions produce
chunks of rhyolite and other volcanic rocks that land near the vent to
form a breccia. As rain or streams wash the debris downslope it is sorted
into layered deposits such as tuff. Finely pulverized rock debris may be
blown high into the atmosphere only to rain down as accretionary lapilli
farther down wind. If such volcanoes really did exist in Maine, they have
been long since eroded. All that remain today are clues preserved in the
rocks that indicate their volcanic origin. |
From the rocks in these photos, we know that this small part of Maine's
crust was formed by volcanic activity much like that which occurs today
in the Caribbean Antilles or the Aegean Sea of the northern Mediterranean.
Unfortunately, the age of the Vinalhaven Rhyolite is difficult to determine,
but the best estimate is that it formed in the Silurian Period, more than
420 million years ago.
Additional information:
Bedrock Geologic History of Maine
Originally published on the web as the March 1998 Site of the Month.
Last updated on March 6, 2008
